Suggested resolution:

Change the grammar in 13.7.2.3 [temp.deduct.guide] paragraph 1 as follows:

deduction-guide:
   explicit-specifieropt template-name ( parameter-declaration-clause ) requires-clauseopt -> simple-template-id ;

The grammar for deduction-guide does not, but should, allow a trailing requires-clause:

deduction-guide:
   explicit-specifieropt template-name ( parameter-declaration-clause ) -> simple-template-id ;